The invention pertains to gas sensors and particularly to toxic gas sensors. More particularly, the invention pertains to micromachined integrated circuit gas and fluid sensors.
Related art devices for sensing toxic gases such as CO, CO.sub.2, NO, NO.sub.2 and VOCs generated by combustion processes have been based on sensors indicating changes in metal oxide film conductivity, chemiluminescence, fluorescence, various forms of IR absorption, and so forth. These sensors have been either too costly, unstable, or insensitive to meet the requirements of a low-cost, reliable toxic gas sensor. Their sensing such toxic gases in concentrations that are commensurate with the levels at which they can be harmful to health and life is difficult, especially if it is to be done via low-cost, affordable and reliable sensors. Often the older gas engines or heaters, operated by budget-minded users, are most likely to be a source of toxic gases which endanger these users and others. These users are the ones most unlikely to buy toxic gas indicators, unless someone manages to bring affordable and appropriate technology to them.
Optoacoustic gas sensors sense low concentrations of gases by inducing a gas temperature variation by narrow-band modulated illumination at a wavelength which the gas absorbs. The modulated temperature signal is not sensed directly, but a closed or nearly-closed gas sample cell is used which converts the small gas temperature signals into a pressure signal, which is detected by a microphone. A closed or nearly-closed gas cell makes it difficult for gas to enter and exit the gas cell.